Reducing sparkle artifacts with low brightness processing

ABSTRACT

A method for reducing sparkle artifacts in a liquid crystal imager, comprises the steps of: low pass filtering only a first lower brightness level signal component of a video signal; and, slew rate limiting only a second lower brightness level signal component of the video signal having the low pass filtered signal component, the video signal having the low pass filtered and the slew rate limited signal components being less likely to result in sparkle artifacts in the imager. Brightness thresholds for defining the lower brightness level signal components and slew rate limits can be selected in accordance with transitions between lower and higher level gain portions of a gamma table associated with the imager, for example an LCOS imager.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of video systems utilizing a liquidcrystal display (LCD), and in particular, to video systems utilizingnormally white liquid crystal on silicon imagers.

2. Description of Related Art

Liquid crystal on silicon (LCOS) can be thought of as one large liquidcrystal formed on a silicon wafer. The silicon wafer is divided into anincremental array of tiny plate electrodes. A tiny incremental region ofthe liquid crystal is influenced by the electric field generated by eachtiny plate and the common plate. Each such tiny plate and correspondingliquid crystal region are together referred to as a cell of the imager.Each cell corresponds to an individually controllable pixel. A commonplate electrode is disposed on the other side of the liquid crystal.Each cell, or pixel, remains lighted with the same intensity until theinput signal is changed, thus acting as a sample and hold. The pixeldoes not decay, as is the case with the phosphors in a cathode ray tube.Each set of common and variable plate electrodes forms an imager. Oneimager is provided for each color, in this case, one imager each forred, green and blue.

It is typical to drive the imager of an LCOS display with aframe-doubled signal to avoid 30 Hz flicker, by sending first a normalframe (positive picture) and then an inverted frame (negative picture)in response to a given input picture. The generation of positive andnegative pictures ensures that each pixel will be written with apositive electric field followed by a negative electric field. Theresulting drive field has a zero DC component, which is necessary toavoid the image sticking, and ultimately, permanent degradation of theimager. It has been determined that the human eye responds to theaverage value of the brightness of the pixels produced by these positiveand negative pictures.

The drive voltages are supplied to plate electrodes on each side of theLCOS array. In the presently preferred LCOS system to which theinventive arrangements pertain, the common plate is always at apotential of about 8 volts. This voltage can be adjustable. Each of theother plates in the array of tiny plates is operated in two voltageranges. For positive pictures, the voltage varies between 0 volts and 8volts. For negative pictures the voltage varies between 8 volts and 16volts.

The light supplied to the imager, and therefore supplied to each cell ofthe imager, is field polarized. Each liquid crystal cell rotates thepolarization of the input light responsive to the root mean square (RMS)value of the electric field applied to the cell by the plate electrodes.Generally speaking, the cells are not responsive to the polarity(positive or negative) of the applied electric field. Rather, thebrightness of each pixel's cell is generally only a function of therotation of the polarization of the light incident on the cell. As apractical matter, however, it has been found that the brightness canvary somewhat between the positive and negative field polarities for thesame polarization rotation of the light. Such variation of thebrightness can cause an undesirable flicker in the displayed picture.

In this embodiment, in the case of either positive or negative pictures,as the field driving the cells approaches a zero electric fieldstrength, corresponding to 8 volts, the closer each cell comes to white,corresponding to a full on condition. Other systems are possible, forexample where the common voltage is set to 0 volts. It will beappreciated that the inventive arrangements taught herein are applicableto all such positive and negative field LCOS imager driving systems.

Pictures are defined as positive pictures when the variable voltageapplied to the tiny plate electrodes is less than the voltage applied tothe common plate electrode, because the higher the tiny plate electrodevoltage, the brighter the pixels. Conversely, pictures are defined asnegative pictures when the variable voltage applied to the tiny plateelectrodes is greater than the voltage applied to the common plateelectrode, because the higher the tiny plate electrode voltage, thedarker the pixels. The designations of pictures as positive or negativeshould not be confused with terms used to distinguish field types ininterlaced video formats.

The present state of the art in LCOS requires the adjustment of thecommon-mode electrode voltage, denoted V_(ITO), to be precisely betweenthe positive and negative field drive for the LCOS. The subscript ITOrefers to the material indium tin oxide. The average balance isnecessary in order to minimize flicker, as well as to prevent aphenomenon known as image sticking.

A light engine having an LCOS imager has a severe non-linearity in thedisplay transfer function, which can be corrected by a digital lookuptable, referred to as a gamma table. The gamma table corrects for thedifferences in gain in the transfer function. Notwithstanding thiscorrection, the strong non-linearity of the LCOS imaging transferfunction for a normally white LCOS imager means that dark areas have avery low light-versus-voltage gain. Thus, at lower brightness levels,adjacent pixels that are only moderately different in brightness need tobe driven by very different voltage levels. This produces a fringingelectrical field having a component orthogonal to the desired field.This orthogonal field produces a brighter than desired pixel, which inturn can produce undesired bright edges on objects. The presence of suchorthogonal fields is denoted disclination. The image artifact caused bydisclination and perceived by the viewer is denoted sparkle. The areasof the picture in which disclination occurs appear to have sparkles oflight over the underlying image. In effect, dark pixels affected bydisclination are too bright, often five times as bright as they shouldbe. Sparkle comes in red, green and blue colors, for each color producedby the imagers. However, the green sparkle is the most evident when theproblem occurs. Accordingly, the image artifact caused bydisclination-is also referred to as the green sparkle problem.

LCOS imaging is a new technology and green sparkle caused bydisclination is a new kind of problem. Various proposed solutions byothers include signal processing the entire luminance component of thepicture, and in so doing, degrade the quality of the entire picture. Thetrade-off for reducing disclination and the resulting sparkle is apicture with virtually no horizontal sharpness at all. Picture detailand sharpness simply cannot be sacrificed in that fashion.

One skilled in the art would expect the sparkle artifact problemattributed to disclination to be addressed and ultimately solved in theimager as that is where the disclination occurs. However, in an emergingtechnology such as LCOS, there simply isn't an opportunity for partiesother than the manufacturer of the LCOS imagers to fix the problem inthe imagers. Moreover, there is no indication that an imager-basedsolution would be applicable to all LCOS imagers. Accordingly, there isan urgent need to provide a solution to this problem that can beimplemented without modifying the LCOS imagers.

BRIEF SUMMARY OF THE INVENTION

The inventive arrangements taught herein solve the problem of sparkle inliquid crystal imagers attributed to disclination without degrading thehigh definition sharpness of the resulting display. Moreover, and absentan opportunity to address the problem by modification of imagers, theinventive arrangements advantageously solve the sparkle problem bymodifying the video signal to be displayed, thus advantageouslypresenting a solution that can be applied to all liquid crystal imagers,including LCOS imagers. Any reduction in detail is advantageously andadjustably limited to dark scenes, even very dark scenes. The videosignal is signal processed in such a way that higher brightness levelinformation is advantageously unchanged, thus retaining high definitiondetail. At the same time, the lower brightness levels of the videosignal that directly result in sparkle are processed in such a way thatthe sparkle is advantageously prevented altogether, or at least, isreduced to a level that cannot be perceived by a viewer. The signalprocessing of the lower brightness level information advantageously doesnot unacceptably degrade the detail of the high definition display.Moreover, signal processing in the form of slew rate limiting canadvantageously be adjusted or calibrated in accordance with thenon-linear gain of any gamma table, and thus, can be used with andadjustably fine tuned for different imagers in different video systems.

In a presently preferred embodiment, the luminance signal of a pictureis decomposed twice into a higher brightness level signal and a lowerbrightness level signal. The demarcation between higher and lowerbrightness levels is adjustable and preferably related to the transitionbetween the lower and higher gain portions of the gamma table. The lowerbrightness levels of the luminance signal are both low pass filtered andslew rate limited.

The lower brightness level signal is low pass filtered after the firstdecomposing to reduce the difference in brightness levels betweenadjacent pixels. The higher brightness level signal is delayed in timeto match the processing delay through the low pass filter. The delaymatched higher brightness level signal and the low pass filtered lowerbrightness level signal are then combined to form an intermediateluminance signal.

The intermediate luminance signal is decomposed into a higher brightnesslevel signal and a lower brightness level signal. The demarcationbetween higher and lower brightness levels is also adjustable and alsopreferably related to the transition between the lower and higher gainportions of the gamma table. The lower brightness level signal is slewrate limited after the second decomposing to limit the difference inbrightness levels between adjacent pixels. The higher brightness levelsignal is delayed in time to match the processing delay through the slewrate limiter. The delay matched higher brightness level signal and theslew rate limited lower brightness level signal are then combined toform a modified output luminance signal.

In a video display system the modified output luminance signal can besupplied to a color space converter, also referred to as a matrix,together with the R-Y and B-Y chrominance signals. The chrominancesignals are also delayed to match the delay through the sparklereduction circuit. The outputs of the color space converter are videodrive signals, for example, R G B, supplied to the LCOS imager. Thesparkle reduction processing changes the brightness levels of the pixelsin the lowest brightness levels, corresponding to the highest gainportions of the gamma table, in such a way as to reduce the occurrenceof declination in the LCOS imager.

A threshold for the luminance signal decomposer, for example, can beexpressed as a digital fraction, for example a digital value of 60 outof a range of 255 digital steps (60/255), as would be present in an8-bit signal. The threshold can also be expressed in IRE, which rangesfrom 0 to 100 in value, 100 IRE representing maximum brightness. The IRElevel can be calculated by multiplying the digital fraction by 100. TheIRE scale is a convenient way to normalize and compare brightness levelsbetween signals having different numbers of bits.

The threshold values for the first and second decomposers, the frequencycharacteristic of the low pass filter and the positive negative slewrate limits can advantageously be selected independently of one another.This enables each one of the constituent components to be related to adifferent sub-portion of the higher gain portion of the gamma table. Inthis respect, the different values can nevertheless advantageously beselected with regard to one another to enable the constituent componentsto act together to provide an optimal result. The following values havebeen selected for the presently preferred embodiment: the thresholdvalue for the first luminance decomposer is 60, corresponding toapproximately 24 IRE for an 8-bit signal; the low pass filter has thefrequency characteristic of a normalized 1:2:1 Z-transform; thethreshold value for the second luminance decomposer is 10, correspondingto approximately 3.9 IRE for an 8-bit signal; and, the positive andnegative slew rates are each limited to one digital value, correspondingto approximately 0.39 IRE for an 8-bit signal. In the presentlypreferred embodiment the frequency characteristic of the low pass filteris fixed and is not thereafter adjustable. Nevertheless, the frequencycharacteristic can advantageously be designed for optimized operationwithin the expected ranges of the selectable threshold values and theselectable slew rate limits.

A method for reducing sparkle artifacts in a liquid crystal imager, inaccordance with the inventive arrangements, comprises the steps of: lowpass filtering only a first lower brightness level signal component of avideo signal; and, slew rate limiting only a second lower brightnesslevel signal component of the video signal having the low pass filteredsignal component, the video signal having the low pass filtered and theslew rate limited signal components being less likely to result insparkle artifacts in the imager.

An apparatus for reducing sparkle artifacts in a liquid crystal imager,in accordance with the inventive arrangements, comprises: means for lowpass filtering only a first lower brightness level signal component of avideo signal; and, means for slew rate limiting only a second lowerbrightness level signal component of the video signal having the lowpass filtered signal component, the video signal having the low passfiltered and the slew rate limited signal components being less likelyto result in sparkle artifacts in the imager.

The inventive arrangements are presently embodied in a two-stage ortandem sparkle reduction process and in a two-stage or tandem sparklereduction processor, as well as video display processing and a videodisplay system. In each of these inventive arrangements, the low passfiltering preferably occurs in the first stage and the slew ratelimiting occurs in the second stage. The low pass filtering preferablyprecedes the slew rate limiting. The use of the noted presentlypreferred values in the sparkle reduction processing of the video signaltaught herein has been found to reduce the sparkle problem for specific,associated LCOS imagers by more than approximately 95%. It will beappreciated by those skilled in the art that other values would likelybe selected to achieve optimum results with other liquid crystal imagersin other liquid crystal video display systems. At present, there is noformula for predicting or calculating optimum values in advance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a sparkle reducing circuit in accordancewith the inventive arrangements.

FIG. 2 is a block diagram useful for explaining the operation ofdecomposers in FIG. 1.

FIG. 3 is a block diagram useful for explaining the operation of a delaymatching circuit and a low pass filter in FIG. 1.

FIG. 4 is a block diagram useful for explaining the operation of a delaymatching circuit and a slew rate limiter in FIG. 1.

FIG. 5 is a block diagram of a portion of a video display systemincorporating different combinations of sparkle reducing circuits.

FIGS. 6( a)–6(e) are waveforms useful for explaining the operation ofthe first stage of the sparkle reducing circuit in FIG. 1.

FIGS. 7( a)–7(e) are waveforms useful for explaining the operation ofthe second stage of the sparkle reducing circuit in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A circuit for reducing sparkle artifacts attributed to disclinationerrors in liquid crystal video systems, for example LCOS video systems,is shown in FIG. 1 and generally denoted by reference numeral 10. Thecircuit comprises a decomposer 12, a slew rate limiter 22, a delay matchcircuit 24 and an algebraic unit 26. An input video signal X, forexample a luminance signal or a video drive signal, is modified by thecircuit 10, and in response, an output video signal X′ is generated. Thevideo signal is a digital signal, and the waveform is a succession ofdigital samples representing brightness levels. The output signal X′ hasa similar digital format. The decomposer 12 generates a higherbrightness level signal 20 and a lower brightness level signal 18. Theoperation of decomposer 12 is illustrated in FIG. 2.

With reference to FIG. 2, a block 14 has a first set of rules forgenerating the higher brightness level signal. The input signal Xrepresents a succession of brightness level samples defining a luminanceinput signal. The brightness level of each sample can be expressednumerically as a digital value or an IRE level, for example 60/255 or 24IRE, as explained above. The letter T represents a threshold value,which can also be expressed as a digital value or an IRE level. If X isgreater than T, then the brightness level H of the higher brightnesslevel signal is equal to X minus T. If X is less than T, then thebrightness level H of the higher brightness level signal is equal to 0.The output of block 14 is denoted HIGH₁ to distinguish from an output ofa second decomposer 30.

A block 16 has a second set of rules for generating the lower brightnesslevel signal. If X is greater than T, then the brightness level L of thelower brightness level signal is equal to the threshold T. If X is lessthan T, then the brightness level L of the lower brightness level signalis equal to X. The output of block 16 is denoted LOW₁ to distinguishfrom an output of the second decomposer 30.

It may be noted that when X=T, the output of block 14 will be the samewhether X is defined as less than or equal to T, or X is defined asgreater than or equal to T. In each case, H is equal to 0. It may alsobe noted that when X=T the output of block 16 will be the same whether Xis defined as less than or equal to T, or X is defined as greater thanor equal to T. In each case, L is equal to X.

Referring again to FIG. 1, the lower brightness level signal 18 is aninput to the low pass filter 22. The higher brightness level signal 20is an input to the delay match circuit 24. The details of the low passfilter 22 and the delay match circuit 24 are shown in FIG. 3. Low passfilter 22 is embodied as a normalized 1:2:1 Z-transform, which is alsodescribed by the formula (1+2Z⁻¹+Z⁻²)/4. The LOW₁ signal is an input toa first one-clock-period delay 221 and a first input to a summingalgebraic unit 222. The output of the first one-clock-period delay is aninput to a second one-clock-period delay 223 and a multiply-by-2algebraic unit 224. The output of the second one-clock-period delay is asecond input to the summing algebraic unit 222. The output of thesumming algebraic unit 222 is a first input to a second summingalgebraic unit 225. The output of the multiply-by-2 algebraic unit is asecond input to the second summing algebraic unit 225. The output of thealgebraic summing unit 225 is an input to a divide-by-4 algebraic unit226. The output of the algebraic unit 226 is LOW_(f).

The low pass filtering incurs a one clock period delay, and accordingly,the delay match circuit 24 provides a one-clock-period delay for thehigher brightness level signal. The low pass filtered lower brightnesslevel signal LOW_(f), and the delayed higher brightness level signalHIGH_(1d) are combined in summing algebraic unit 26, which generates theintermediate signal X′.

The second stage 10B comprises the second decomposer 30, a slew ratelimiter 36, a second delay match circuit 38 and a second algebraic unit40. The input signal X′ is the intermediate signal generated by thefirst stage 10A. The output is a modified luminance signal X″. Thedecomposer 30 generates a higher brightness level signal 34 and a lowerbrightness level signal 32. The operation of decomposer 30 is the sameas explained in connection with decomposer 12 illustrated in FIG. 2. Thesame sets of rules apply, but as noted, the threshold value T need notbe the same in decomposers 12 and 30. The outputs of blocks 14 and 16are denoted HIGH₂ and LOW₂ to distinguish from the outputs of the firstdecomposer 12.

Referring again to FIG. 1, the lower brightness level signal LOW₂ online 32 is an input to the slew rate limiter 36. The higher brightnesslevel signal HIGH₂ on line 34 is an input to the delay match circuit 38.The details of the slew rate limiter 36 and the delay match circuit 38are shown in FIG. 4. Slew rate limiter 36 assures that successive outputsignals from the slew rate limiter will not vary by more than thepredetermined slew rate. The decomposed LOW₂ signal is an input to analgebraic unit 361. The other input to the algebraic unit 361 is thepreceding output of the slew rate limiter stored in latch 372. The lastoutput value LOW_(s) is subtracted from the input value to determine thedifference. The difference on output line 362 is an input to a firstcomparator 364 denoted MIN and a second comparator 365 denoted MAX. Thedifference is tested in the MIN circuit to see if the difference isgreater than a positive slew limit S and is also tested in the MAXcircuit to see if the difference is more negative than the negative slewlimit −S. It is not necessary that the positive and negative slew limitshave the same absolute value, although the same absolute value is usedin the embodiment shown in FIG. 4.

The most significant bit (MSB) of the difference signal 362 is thecontrol input to a multiplexer (MUX) 368. The most significant bit ofthe difference indicates the polarity of the difference and selects theoutput 366 of comparator 364 or the output 367 of comparator 365. Theoutput of the MIN comparator is selected when the difference is positiveand the output of the MAX comparator is selected when the difference isnegative. The output of the multiplexer on line 369 is a slew ratelimited difference that is added to the brightness level of the previousslew rate limited output pixel in algebraic unit 370, in order togenerate the next new pixel. The output of the algebraic unit 370 online 371 is stored in the latch 372. The LOW_(s) output of the latch 372is a stream of slew rate limited pixels. The embodiment of the slew ratelimiter shown in FIG. 4 incurs a one-pixel delay, even if the slew rateis not limited. The clock signals are omitted from FIG. 4 for purposesof clarity.

Although the positive and negative slew rates in the example shown inFIG. 4 have the same absolute value, this need not be the case.Advantageously, the slew rates can be set independently for samplevalues greater than the preceding sample value and for sample valuesless than the preceding pixel value. If the positive and negative slewrates are equal to 1, for example, then successive outputs of the slewrate limiter will not differ from one another by more than 1 digitalvalue step. If the LOW_(s) signal has an 8-bit value, then successiveoutputs of the slew rate limiter will not differ from one another bymore than one step out of 256 states, corresponding to approximately0.39 IRE.

The one pixel delay of the slew rate limiter corresponds to a one clockperiod delay, and accordingly, the delay match circuit 38 provides aone-clock-period delay for the higher brightness level signal. It ispossible under some circumstances that the delay incurred by the slewrate limiter can exceed a one-clock-period delay, but the delay matchcircuit need not be adjusted accordingly. The slew rate limited lowerbrightness level signal LOW_(s) and the delayed higher brightness levelsignal HIGH_(2d) are combined in the algebraic unit 40, which generatesthe output signal X″.

The first and second lower brightness level signal components canadvantageously be defined by selecting different brightness thresholdsin accordance with transitions between lower and higher level gainportions of a gamma table associated with the LCOS imager. Slew ratelimits can advantageously be selected in accordance with the lower andhigher level gain portions of the gamma table. The response of differentstages of the sparkle reduction processing, and processor, can thereforeadvantageously be adjusted in accordance with different sub-portions ofthe higher gain portion of the gamma table.

A video system 50 shown in FIG. 4 illustrates various combinations inwhich video signals, for example luminance signals and video drivesignals, can be processed for sparkle reduction. A color spaceconverter, or matrix, 52 generates video drive signals, for example RGB,responsive to a luminance signal, denoted LUMA, and chrominance signals,denoted CHROMA. The chrominance signals are more particularly designatedR-Y and B-Y.

Two sets of inputs to the color space converter 52 are denoted 54A and54B. In set 54A the LUMA signal input is modified by sparkle reductionprocessor (SRP) 10 to generate LUMA″. The CHROMA signals are delayed bydelay match (DM) circuits 56. The delay match is two clock periods, onefor the first stage 10A and one for the second stage 10B. In set 54B theLUMA signal is not modified and the CHROMA signals are not delaymatched.

Four sets of outputs from the color space converter 52 are denoted 60A,60B, 60C and 60D. In set 60A the video drive signals RGB are notmodified. In set 40B, each one of the RGB video drive signals ismodified by a sparkle reduction processor 10 to generate R″, G″ and B″.No delay matching is necessary. In set 60C only one of the video drivesignals, for example G, is modified by sparkle reduction processor 10 togenerate G″. The remaining video drive signals are delayed by delaymatching circuits 56. This delay match is also two-clock-periods. In set60D only two of the video drive signals, for example R and G, aremodified by sparkle reduction processors 10 to generate R″ and G″. Theremaining video drive signal is delayed by delay matching circuit 56.Input set 54A can be used with any one of output sets 60A, 60B, 60C or60D. Input set 54B can be used with any one of output sets 60B, 60C or60D. The combination of input set 54B and output set 60A does notinclude sparkle reduction processing.

It has been found that using the combination of input set 54A and outputset 60A reduces the sparkle artifact attributed to declination by morethan approximately 95%. It should be remembered that the thresholdvalues for each of the decomposers, the frequency characteristic of thefilter and the slew rate limits can advantageously be independentlyselected. This enables the sparkle reduction processing to be fine tunedto different LCOS imagers in different video display systems.

The response of the first stage of circuit 10 in FIG. 1 to a specificinput signal is illustrated in FIGS. 6( a) through 6(e). For purposes ofillustration, the threshold T is set to the digital value or state of60, corresponding to approximately 24 IRE for an 8-bit signal. The lowpass filter operates according to the normalized 1:2:1 Z-transform. Theresponse of the second stage of circuit 10 in FIG. 1 to the output ofthe first stage is illustrated in FIGS. 7( a) through 7(e). For purposesof illustration, the threshold T is set to the digital value or state of10, corresponding to approximately 3.9 IRE for an 8-bit signal. Forpurposes of illustration, the positive and negative slew rates are eachlimited to one digital state or value, corresponding to approximately0.39 IRE for an 8-bit signal.

It should be noted that the vertical scales for FIGS. 6( a) through 6(e)are not all the same. The value of the brightness level is equal to aclearly discernable value along the y-axis, except in FIG. 6( e), whereeach sample is also accompanied by its digital value. None of the y-axisscales is the same in FIGS. 7( a) through 7(e). The values for eachsample are clearly discernable except in FIGS. 7( a) and 7(e), whereeach sample is also accompanied by its digital value. It should also benoted that FIGS. 6( e) and 7(a) are the same waveform.

In FIG. 6( a), stage 10A has an input signal X having the luminancevalues shown by the black dots. Each black dot represents a sample of aluminance value as an input to the decomposer 12. Each sample representsthe brightness level of a pixel. The signal X can be seen as including apulse followed by an impulse. The threshold value of T, as explained inconnection with the rules of FIG. 2, is equal to 60 in this example.

The first two values of X are 0. In accordance with block 14, the valueof the delay matched higher brightness level signal HIGH_(1d) in FIG. 6(b) is 0 because X is less than T. The next three input values are 80.The corresponding levels of the higher brightness level signal in FIG.6( b), which is the HIGH_(1d) output of the delay match circuit, are 20because the output value equals the input value minus the thresholdvalue (X−T). The remaining sample values are calculated in the samefashion.

With reference to FIG. 6( c), the first two output values of the lowerbrightness level signal LOW₁ are 0, because the input is less than thethreshold and the output equals the input. The next three output valuesare equal to 60 because the input value is greater than that threshold,and for LOW₁, the output equals the threshold value. The remainingsamples are calculated in the same fashion.

FIG. 6( d) represents the output LOW_(f) of low pass filter 22responsive to the signal shown in FIG. 6( c). It can be noted that thepulse and impulse which are still evident in the waveform of FIG. 6( c)have been considerably smoothed, or rolled off, by the low passfiltering.

FIG. 6( e) is the intermediate signal X′, which is the sum of thewaveforms in FIGS. 6( b) and 6(d). The values of each sample other than0 are noted. It can be seen from the waveform in FIG. 6( e) that theessential character of the pulse and of the impulse in the inputwaveform X been retained in the intermediate waveform X′, but sharpedges or transitions between adjacent sample values have beenadvantageously reduced.

The response of the second stage 10B of circuit 10 in FIG. 1 to theintermediate waveform X′ is illustrated in FIGS. 7( a) through 7(e). Thethreshold value of T for the second decomposer 30 is equal to 10,corresponding to approximately 3.9 IRE. The slew rate limits of the slewrate limiter 36 are set to 1 and to −1. Accordingly, successive outputsamples of the slew rate limiter cannot vary from one another by morethan one digital state or step, approximately 0.39 IRE.

FIG. 7( a) illustrates the same intermediate waveform X′ as in FIG. 6(e). The first value of X′ is 0. In accordance with block 14, the valueof the higher brightness level signal in FIG. 7( b), which is the outputHIGH_(2d) of the delay match circuit 38, is 0 because X is less than T.The next five input values are greater than the threshold T, so thateach corresponding output value is calculated by subtracting 10 from thebrightness level in FIG. 7( a). Where X′ is 15, for example, HIGH₂ is 5.Where X′ is 65, for example, HIGH₂ is 55. The following values ofHIGH_(2d) are 70, 55, 5, 0, 5, 40 and 5.

With reference to FIG. 7( c), the first output value of the lowerbrightness level signal LOW₂ is 0, because the input is less than thethreshold and the output equals the input. The next five output valuesare equal to 10 because the input value is greater than the threshold,so the output equals the threshold value. The remaining samples arecalculated in the same fashion.

FIG. 7( d) represents the output LOW_(s) of slew rate limiter 36responsive to the signal shown in FIG. 7( c). The value of the firstsample of LOW₂ is 0. Since 0 is less than 1, the values of the firstsamples of LOW_(s) is 0. The value of the second sample of LOW₂ is 10.Since 10 exceeds 0 by more than the slew limit of 1, the value of thesecond sample of LOW_(s) is 1. The value of the third sample of LOW₂ is10. Since 10 exceeds 1 by more than the slew limit of 1, the value ofthe third sample of LOW_(s) is 2. The value of the fourth sample of LOW₂is 10. Since 10 exceeds 2 by more than the slew limit of 1, the value ofthe fourth sample of LOW_(s) is 3. The value of the fifth sample of LOW₂is 10. Since 10 exceeds 3 by more than the slew limit of 1, the value ofthe fifth sample of LOW_(s) is 4. The value of the sixth sample of LOW₂is 10. Since 10 exceeds 4 by more than the slew limit of 1, the value ofthe sixth sample of LOW_(s) is 5. The value of the seventh sample ofLOW₂ is 0. Since 0 is less than 5 by more than the slew limit of 1, thevalue of the seventh sample of LOW_(s) is 4. The value of the eighthsample of LOW₂ is 10. Since 10 exceeds 4 by more than the slew limit of1, the value of the eighth sample of LOW_(s) is 5. The value of theninth sample of LOW₂ is 10. Since 10 exceeds 5 by more than the slewlimit of 1, the value of the ninth sample of LOW_(s) is 6. The remainingvalues can be calculated in a similar manner. The effect of the slewrate limiting on smoothing or rolling off the slopes and transitions ofthe waveform in FIG. 7( c) is evident.

Finally, FIG. 7( e) is the output signal X″, which is the sum of thewaveforms in FIGS. 7( b) and 7(d). It can be noted from the waveform inFIG. 7( e) that the essential character of the pulse and of the impulsein the input waveform X been retained in the output waveform X″ afterthe tandem processing, but sharp edges or transitions between adjacentsample values have been advantageously reduced. The maximum amplitudesof the pulse and impulse have also been advantageously reduced, andthere are a smaller number of zero values as well. Only the very darkareas of the picture are noticeably affected by the sparkle reductionprocessing, as evidenced by the very low IRE values of the decomposerthresholds and the slew rate limits. Accordingly, the high definitionhorizontal resolution is advantageously maintained.

The methods and apparatus illustrated herein teach how the brightnesslevels of adjacent pixels can be restricted or limited in the horizontaldirection, and indeed, these methods and apparatus solve the sparkleproblem. Nevertheless, these methods and apparatus can also be extendedto restricting or limiting brightness levels of adjacent pixels in thevertical direction, or in both the horizontal and vertical directions.

1. A method for reducing sparkle artifacts due to non linearity in atransfer function of a liquid crystal imager comprising the steps of:decomposing an input video signal according to a first threshold toprovide a first low brightness signal; low pass filtering said first lowbrightness signal to provide a low pass filtered low brightness signal;slew rate limiting said low pass filtered low brightness signal; andsaid providing an output video signal including said low pass filteredslew rate limited signal, said output video signal being less likely toresult in sparkle artifacts in said imager.
 2. The method of claim 1wherein said decomposing step provides a first high brightness signalaccording to said first threshold, the method comprising the step ofcombining said low pass filtered first brightness signal and said highbrightness signal prior to said slew rate limiting.
 3. The method ofclaim 2 comprising the step of delay matching said first high brightnesssignal with said low pass filtered low brightness signal prior to saidcombining step.
 4. The method of claim 2 comprising the steps of:decomposing said combined signal into a second low brightness signal anda high brightness signal according to a second threshold prior to saidslew rate limiting; applying said slew rate limiting step to said secondlow brightness signal; and, combining said slew rate limited second lowbrightness signal and said second high brightness signal to generatesaid output video signal.
 5. The method of claim 4 comprising the stepof delay matching said second high brightness signal with said slew ratelimited low brightness signal prior to said combining step.
 6. Themethod of claim 1 comprising the step of supplying said output videosignal to a liquid crystal on silicon imager.
 7. The method of claim 1comprising the steps of: applying said sparkle reducing steps to aninput video signal comprising a luminance signal for a picture; delayingchrominance signals for said picture; and, generating a plurality ofvideo drive signals from said processed luminance signal and saiddelayed chrominance signals.
 8. The method of claim 7 comprising thesteps of: applying said sparkle reducing steps to further process atleast one of said video drive signals; and, delaying non-sparkle-reducedvideo drive signals.
 9. The method of claim 1 comprising the steps of:generating a plurality of video drive signals from luminance andchrominance signals; applying said sparkle reducing steps to at leastone of said video drive signals; and, delaying non-sparkle-reduced videodrive signals.
 10. The method of claim 4 comprising the steps of:selecting said first and second thresholds in accordance withtransitions of a gamma table associated with said LCOS imager; and,selecting slew rate limits in accordance with the gain of said gammatable.
 11. An apparatus for reducing sparkle artifacts due to nonlinearity in a transfer function of a liquid crystal imager comprising:means for decomposing an input video signal according to a firstthreshold to provide a first low brightness signal; means for low passfiltering said first low brightness signal to provide a low passfiltered low brightness signal; means for slew rate limiting said lowpass filtered low brightness signal; and means for providing an outputvideo signal including said low pass filtered slew rate limited signalto said imager.
 12. The apparatus of claim 11 wherein said decomposerprovides a first high brightness signal in accordance with said firstthreshold, said apparatus comprising; first means for combining said lowpass filtered low brightness signal and said first high brightnesssignal prior to said slew rate limiting; means for dividing saidcombined signal into a second low brightness signal and a second highbrightness signal prior to said slew rate limiting; and, second meansfor combining said slew rate limited second low brightness signal andsaid second high brightness signal to generate said output video signal.13. The apparatus of claim 12 comprising: means for delay matching saidfirst high brightness signal with said low pass filtered low brightnesssignal; and, means for delay matching said second high brightness signalwith said slew rate limited second low brightness signal.
 14. Theapparatus of claim 11 wherein a picture to be displayed on said imagerincludes luminance signals comprising said input signal and chrominancesignals, the apparatus comprising: means for delaying said chrominancesignals for said picture; and, means for generating a plurality of videodrive signals based upon said output video signal and said delayedchrominance signals.
 15. The apparatus of claim 11 wherein: said firstand second thresholds are selectable in accordance with transitionsbetween lower and higher gain portions of a gamma table associated withsaid imager; and, slew rate limits are selectable in accordance with thegain of said gamma table.
 16. The apparatus of claim 11 wherein saidmeans for low pass filtering has a normalized 1:2:1Z-transform frequencycharacteristic.
 17. The apparatus of claim 11 wherein said imagercomprises a liquid crystal on silicon imager.
 18. An apparatus forreducing sparkle artifacts due to non linearity in a transfer functionof a liquid crystal imager comprising: a decomposer for providing afirst low brightness signal in accordance with a first threshold; a lowpass filter for filtering said first low brightness signal to provide afiltered low brightness signal; and, a slew rate limiter for processingsaid filtered low brightness signal to provide an output video signalless likely to result in sparkle artifacts in said imager.
 19. Theapparatus of claim 18 wherein said first decomposer provides a firsthigh brightness signal according to said first threshold said apparatuscomprising: a first algebraic unit for combining said low pass filteredlow brightness signal and said first high brightness signal prior toprocessing by said slew rate limiter; a second decomposer for dividingsaid combined signals in accordance with a second threshold into asecond low brightness signal and a second high brightness signal priorto processing by said slew rate limiter; and, a second algebraic unitfor combining said slew rate limited second low brightness signal andsaid second high brightness signal to generate said output video signal.20. The apparatus of claim 19 comprising: a first delay match circuitfor delaying said first high brightness signal prior to said combiningwith said low pass filtered first low brightness signal; and, a seconddelay match circuit for delaying said second high brightness signalprior to said combining with said slew rate limited second lowbrightness signal.
 21. The apparatus of claim 20 wherein a picture to bedisplayed on said imager comprises a luminance and chrominance signaland said input signal comprises said luminance signal the apparatuscomprising: a delay matching circuit for delaying chrominance signal;and, a color space converter for generating a plurality of video drivesignals based upon said output signal and said delayed chrominancesignals.
 22. The apparatus of claim 18 wherein said first and secondbrightness thresholds are selectable in accordance with transitionsbetween relatively lower and higher gain portions of a gamma tableassociated with said imager and slew rate limits are selectable inaccordance the gain of said gamma table.
 23. The apparatus of claim 20wherein said low pass filter has a normalized 1:2:1Z-transform frequencycharacteristic.
 24. The apparatus of claim 20 wherein said imagercomprises a liquid crystal on silicon imager.